The invention relates to a display device comprising an electro-optical medium which is switchable between two optical states and is arranged between a first supporting plate provided with row electrodes and a second supporting plate provided with column electrodes divided into n column sub-electrodes (where n.gtoreq.4), at least two of which have different widths and which define n sub-pixels at the area of a crossing with a row electrode, the device having a drive circuit for energizing combinations of column sub-electrodes associated with grey scale stages.
Such an electro-optical medium usually switches between two optical states with a steep transition characteristic (transmission/voltage characteristic curve) or, in the case of, for example, liquid crystal display devices (such as supertwist display devices or ferro-electrical display devices) with a hysteresis in this transition characteristic.
The two optical states (possibly together with polarizers and/or reflectors) define two extreme transmission levels which represent the extremes of a grey scale.
A display device of the type described in the opening paragraph is described in EP-A-0 316 774. The display device is driven in the multiplex mode, i.e., by consecutively energizing address lines (row electrodes) while the information to be written is being presented on data lines (column electrodes). Intermediate levels (grey scale stages) can be represented in such a display device by dividing the column electrodes into sub-electrodes having different surface areas (for example, in accordance with surface area ratios of 8:4:2:1).
With such an exponential subdivision (2.sup.p :2.sup.p-1.. . . :2:1) a maximum number of grey scale stages (levels) can be selected, namely 2.sup.n, including fully on and fully off, with a minimum number of connections of the sub-electrodes n per column. This number can be increased by also subdividing the selection (row) electrodes or by using a weighted drive.
The allocation of column sub-electrodes to be switched on is unambiguously coupled to a given grey scale stage by the exponential division of the sub-electrodes. However, the number of variations, i.e. the number of sub-pixels switching on or switching off upon transition to a next higher or next lower grey scale stage is then also fixed.
This may mean that large parts of the pixel change their optical stage in the case of such transitions. For example, for a pixel having a width ratio of 8:4:2:1 of the sub-columns, in an extreme case a transition may occur in which the widest sub-column switches from light to dark, whereas the other sub-columns switch from dark to light. In some applications, notably in projection television, such transitions as well as less extreme transitions are visible as artifacts in the image, at the recommended viewing distance (approximately 6 times the image width) and even further.
To indicate a criterion for the extent of change permissible in the case of such a transition, we refer to the change of periodicity. Periodicity is understood to mean the display, translated to amplitude and phase, of a fundamental wave related to the light/dark division across the pixel, as will be explained further hereinafter. Viewed across the width of a pixel, the transmission or reflection is to this end represented by a block function having, for example, the value of 1 for light parts and the value of 0 for dark parts. With the change described above, this function acquires a complementary value throughout the width of the pixel, and the change of periodicity is maximal.
A possible way of reducing the visibility of transitions at the viewing distance is to subdivide the column into a large number of, for example 15 sub-electrodes of equal width and to introduce the stages (levels) by starting with one sub-electrode and by switching on an adjoining sub-electrode for each subsequent stage. However, this is at the expense of the number of connections; to realize 16 stages, including fully on and fully off, 15 connections instead of 4 are then required.